Gold Elution Column

Overview

A gold elution column reverses the step that loaded gold onto activated carbon. In a carbon-in-pulp or carbon-in-leach plant the dissolved gold cyanide complex adsorbs onto carbon as the carbon moves countercurrent to the slurry. The loaded carbon, carrying perhaps several thousand grams of gold per tonne, is screened out and moved to this column. Here hot caustic and cyanide solution is pumped up through the packed carbon bed and pulls the gold back off the carbon into a concentrated liquid called eluate or pregnant strip solution. That eluate then goes to electrowinning, where the gold is plated out. The stripped carbon is washed, acid treated, and reactivated for reuse.

The column is run as a batch. A measured charge of loaded carbon is loaded into the Column Vessel, the bed is sealed, and hot eluant is circulated through it under pressure for several hours. Pressure keeps the solution liquid above its atmospheric boiling point so the bed can run at 110-130 C, where elution is fast. Two process families dominate: the AARL method soaks the bed in a strong caustic cyanide pre-soak then displaces it with hot water, while the Zadra method recirculates a single caustic cyanide solution continuously between the column and the electrowinning cells.

The column vessel

The Column Vessel is a vertical stainless pressure vessel. Its Pressure Shell is rolled from 304L or 316 plate and rated for the full operating pressure and temperature with margin. Carbon is held in the bed by a Top Retention Screen at the outlet and spread evenly by a Bottom Distributor Screen at the inlet, both wedge-wire so they pass solution but not carbon granules. The ends close with bolted Vessel Flange joints sealed by an O-Ring Set, which open to load fresh carbon and dump stripped carbon. A Insulation Jacket blanket under cladding holds the heat in, and a Relief Valve protects the shell if pressure climbs past its rating. Running the bed upflow keeps the carbon fluidised slightly and prevents channeling, so eluant contacts every granule.

Heating and heat recovery

Bringing tonnes of solution to 125 C every cycle is the largest energy cost, so the column pairs a Solution Heater with a Heat Exchanger Train. The heater is an electric flow heater built around several sheathed Heating Element units inside a Heater Vessel; a Thermocouple trims the power and a High-Limit Thermostat cuts the elements on overtemperature through the Heater Contactor. The heat train recovers most of that energy: a Recovery Exchanger passes hot barren eluate leaving the column against cold rich eluate entering it, recovering 60-75 percent of the sensible heat. A smaller Trim Exchanger closes the gap to set point, and a Eluate Cooler drops the barren stream before it returns to the leach circuit. The plates clamp in the Exchanger Frame.

Pumping and reagent dosing

The Eluate Pump Skid moves solution through the bed and the train against the pressure drop of packed carbon and wedge-wire screens. Its Eluate Pump is a multistage centrifugal unit whose Pump Impeller stages build head inside a stainless Pump Casing, with a Mechanical Seal holding the hot caustic in and a Servo Motor driving the shaft on Ball Bearing supports. A Transfer Pump shuttles rich and barren eluate between the column and storage. Eluant strength is set on the Reagent Dosing Skid, where a Caustic Metering Pump and Cyanide Metering Pump meter sodium hydroxide and sodium cyanide into the Make-up Tank, kept blended by the Tank Mixer and watched by the Level Sensor.

Controls and piping

The Control Panel runs the whole batch. Its PLC Controller times the pre-soak, elution and water-wash stages, holds temperature and flow at set point, and trips on the Pressure Sensor readings and circuit thermocouples. An operator works the cycle from an LCD Panel, with Relay interlocks and a Power Supply feeding the loop. Solution routing runs through the Piping Manifold manifold, where Control Valve units the PLC modulates set the flow and temperature splits, Isolation Valve units block off equipment for carbon transfer, and a Flow Meter confirms bed velocity through the Pipe Spool runs.

Operation and variants

A strip cycle runs 8-14 hours depending on carbon loading, temperature and method. AARL columns give a small, very concentrated eluate batch ideal for fast electrowinning, at the cost of fresh reagent each cycle and a water-wash step. Zadra columns recirculate one solution continuously through the cells, which is simpler and cheaper on reagent but produces a more dilute eluate and runs longer. Column size scales with plant throughput, from one-tonne units in small heap operations to twelve-tonne columns in large tank-leach plants. After elution the carbon is acid washed to remove carbonate scale, reactivated in a kiln, and returned to the adsorption circuit, so the column sits at the heart of a closed carbon loop that a mine runs continuously for the life of the operation.